The exploration and production of hydrocarbons from subsurface formations ultimately requires a method to reach and extract the hydrocarbons from the formation. This is typically achieved by drilling a well with a drilling rig. In its simplest form, this constitutes a land-based drilling rig that is used to support and rotate a drill string, comprised of a series of drill tubulars with a drill bit mounted at the end. Furthermore, a pumping system is used to circulate a fluid, comprised of a base fluid, typically water or oil, and various additives down the drill string, the fluid then exits through the rotating drill bit and flows back to surface via the annular space formed between the borehole wall and the drill bit. After being circulated through the bore hole, the drilling fluid normally flows back into a mud handling system, generally comprised of a shaker table, to remove solids, a mud pit and a manual or automatic means for addition of various chemicals or additives to keep the properties of the returned fluid as required for the drilling operation. Once the fluid has been treated, it can be circulated back into the bore hole via re-injection into the top of the drill string with the pumping system.
During drilling operations, the fluid exerts a pressure against the bore hole wall that is mainly built-up of a hydrostatic part, related to the weight of the mud column, and a dynamic part related frictional pressure losses caused by, for instance, the fluid circulation rate or movement of the drill string.
However, in some geological systems, the formation has many natural fractures and/or is extremely permeable. Consequently, (large quantities of) drilling fluid is lost in formation fractures during circulation of drilling fluid.
Sometimes, an effect known as “formation breathing” occurs, whereby the formation returns fluid when pumping of fresh drilling fluid into the hole is interrupted, mostly of a different density than the original drilling fluid. This results in kicks, a well control problem, often resulting in a lost hole section or well. During the planning phase of wells, the expectation of severe formation breathing may result in cancelling the well based on risk analysis.
A quantity of the drilling fluid may, however, remain behind in the formation.
One way of coping with such loss of circulation fluid is to accept the losses and drill ahead. This is known as “blind drilling”, “floating drilling”, “mudcap drilling”, or “closed hole circulation drilling”. A clean and preferably cheap drilling fluid would be pumped down the drill string, to be lost into the formation. To control the reservoir, overbalanced mud would be pumped into the annular space at a rate that is higher than the hydrocarbon migration rate. The well control capabilities are quite limited and for safety reasons the application of “blind drilling” has thus been limited to low pressured and/or non-sour formations.